Though significant effort has been expended to “tissue-engineer” natural grafts for the replacement of damaged or diseased load-bearing extracellular matrices (ECMs) such as ligament, tendon and cornea, no clinically viable constructs have been produced. One of the most difficult problems associated with engineering connective tissue (which typically begins with weak, self-assembled collagen-based scaffolds) is the achievement of adequate mechanical strength. The inability of such constructs to bear in vivo or near in vivo loads at the time of implantation has led to the development of methods which employ complex degradable high-strength scaffolds (e.g., silk) onto which fibroblastic cells are seeded. Once seeded, the fibroblasts in the matrix are stimulated (chemically and/or mechanically) to replace the resorbing scaffold material with a natural collagen-based matrix. This approach is hampered by uncertainties in the kinetics of the degradation and replacement process as well as the usual problems associated with the use of non-native biomaterials.
Collagen molecules comprise an epigenetically adaptable load-bearing matrix that can be culled by enzymatic degradation or reinforced by incorporation of monomers (longitudinally and in the radial direction). The generic ability of collagen-based matrices to add and remove monomers from fibrils is a remarkable feature of native load-bearing ECM, as are its mechanical properties (due to collagen's high tensile mechanical strength). Such tissue can be characterized by its highly anisotropic and ordered collagen fibrillar organization (cornea (Hay et al., Monogr. Dev. Biol. 1:1-144 (1969)), ligament and tendon (Provenzano et al., Matrix Biol. 25:71-84 (2006)), and annulus fibrosus (Marchini et al., Basic Appl. Histochem. 23:137-148 (1979)). Unfortunately, collagen has limited regenerative ability following injury (Fini et al., Cornea 24:S2-S11 (2005); Frank, Musculoskelet. Neuronal Interact. 4:199-201 (2004); Lotz et al., Neurosurg. Clin. N. Am. 16:657-663 (2005)).
Fibrillar collagen is the principal load-bearing molecule in vertebrate animals and the most abundant protein in vertebrates. The untimely failure of collagenous load-bearing tissues (which are often refractory to self-repair) due to degeneration, acute injury or collagen-related disease affects hundreds of millions of people world-wide and often can have a devastating impact on the quality of life of the individual if left untreated. In the industrialized world, degenerative disease of collagen-based tissue has a high prevalence. In the US, 32 million people over the age of 20 have frequent lower back pain; the majority of these cases are likely the result of intervertebral disc degeneration. The cost of indirect and direct medical care for herniated disks was estimated to be $1.6B in 1995. Osteoarthritis, a degenerative cartilage pathology of unknown etiology, affects over 20 million US adults and is second only to chronic heart disease as the reason for long-term disability payment requests. Acute injury to connective tissue also contributes significantly to the loss of load-bearing tissue function. There are 200,000 anterior cruciate ligament (ACL) injuries and between 60,000 and 95,000 ACL reconstructions are performed each year in the US to restore mechanical function.
There also are a constellation of collagen-related diseases due to genetic mutations which include Ehlers-Danlos syndrome, Bethlem myopathy, Alport syndrome, Knobloch syndrome, osteoporosis (some cases), osteogenesis imperfecta, arterial aneurysm and rheumatoid arthritis (autoimmune).
Very little is known about the mechanisms which govern the organization and morphology of collagen during synthesis by fibroblasts in vivo, for it is the loss of or damage to organized collagen that is often irreparable. The majority of tissue engineers have opted to investigate synthetic biomaterials and have, with few exceptions, treated collagen as a “degradable” cell transport vehicle. There are more than 10 million cases of corneal blindness (caused by both injury and disease), the vast majority of which could benefit from suitable corneal replacement. In the US, 33,000 corneal transplants are performed each year in the US; however, recipients will be subject to a looming graft material shortage induced by the extensive use of LASIK corrective surgery, which renders corneas unsuitable for donation.